1. Field of the Invention
The present invention relates to a method and system for monitoring an optical transmission line, and more particularly, to a method and system for monitoring an optical transmission line by using an optical time-domain reflectometer (OTDR) and an optical amplifier.
2. Description of Related Art
The optical time domain reflectometry (OTDR) is very convenient to monitor WDM transmission links. In order to solve problems in the transmission line quickly and further to prevent potential problems, online monitoring at a central office is necessary. However, the monitoring distance with a typical OTDR is limited to about 160 km of optical fiber because of the dynamic range of avalanche photo diode (˜35 dB). To further extend the monitoring length, amplified OTDR systems using optical amplifiers have been investigated, and then, the defective positions of the optical transmission line and the loss of transmission measured by the OTDR are used to improve reliability of an optical transmission system.
Typically, amplification of the monitoring light of the OTDR results in backward amplified spontaneous emission (ASE), which is generated by the optical amplifier. In turn, the ASE may deteriorate performance of the OTDR. In an optical transmission line monitoring system with multiple amplification stages, the performance of the OTDR may be greatly deteriorated because of ASE components outside a wavelength band of the monitoring light. Therefore, it is necessary to remove the ASE components outside a wavelength band of the monitoring light.
Since the monitoring light of the OTDR is a pulsed light, a signal input to the optical amplifier is periodically in on and off states. When the input signal of the optical amplifier is in the off state, the ASE from optical amplifier greatly increases. Therefore, there is a difference in the power of ASE between the on and off states of the input signal. If a pumping operation for increasing gain of the optical amplifier is intensively performed, the difference in the ASE may be further enlarged. Since the ASE generated in the off state of the input signal becomes a background noise, the operating characteristics of photo diodes in the receiver of the OTDR may be deteriorated. In addition, results of monitoring will be distorted
FIG. 1 is a graph showing optical spectra of outputs of an optical amplifier with or without a signal
The ASE without a signal is larger than that with a signal. Because of the difference in the ASE, the performance of the OTDR will be deteriorated.